Abstract Tumors are known as wounds that do not heal - implying that cells involved in angiogenesis and injury response, such as endothelial cells and fibroblasts, have a prominent role in the progression, growth and spread of cancers. Our knowledge regarding the role of the resting and the activated fibroblasts (also known as carcinoma-associated fibroblasts, CAF) in cancer is still evolving. Fibroblast production of growth factors, chemokines and extracellular matrix likely facilitates the angiogenic recruitment of endothelial cells and pericytes. Moreover, the precise role of fibroblasts in the recruitment and the formation of metastasis is completely unknown. Therefore, this grant application is focused on the role of fibroblast in cancer progression, with specific emphasis on metastasis. Emerging data from other laboratories and our own preliminary data suggest that fibroblasts are a heterogeneous population. In the tumor microenvironment, we have now identified a population of CAF that are positive for fibroblast specific protein-1 (FSP1) but not for alpha-smooth muscle actin (alphaSMA). Our studies demonstrate for the first time that not all CAF are alpha SMA+. Additionally, specific ablation of FSP1+/alphaSMA- fibroblasts using transgenic mice, which harbor FSP1 promoter driven thymidine kinase protein (FSP1-tk), demonstrate that FSP1+ fibroblasts are critical for the formation of metastasis without a significant impact on the growth of angiogenic primary tumors. Furthermore, we demonstrate that VEGF, SDF-1 and Tenascin-C, might mediate the formation of metastatic nodules.To further evaluate the role of these fibroblasts in the formation of metastasis, we propose the following aims in this proposal: 1. To evaluate the contribution of FSP1+ fibroblasts in the growth of primary tumors and recruitment of metastasis, 2. To address the contribution of VEGF produced by FSP1+ fibroblasts in the growth of primary tumors and the formation of metastasis, 3. To investigate the contribution of SDF-1, produced by FSP1+ fibroblasts, a downstream effector of VEGF, in the formation of metastasis, and 4. To study the contribution of Tenascin-C, a FSP1+ fibroblast-produced extracellular matrix protein, in the organization of metastasis nodules. Successful completion of experiments proposed in this grant application will provide crucial insights into the role of FSP1+ fibroblasts as key determinants for the organization of metastasis. Narrative Cancer cells are not the only constituent cells within a primary tumor and secondary metastasis. Other cells including fibroblasts are found inside the primary tumor and secondary metastatic nodules. The role of such fibroblasts in cancer progression and metastasis is unknown. This proposal addresses the role of fibroblasts in facilitating metastasis.